An A/D converter circuit for converting an analog voltage signal input to an input signal line into a binary digital data and outputting it to an output data line is disclosed in, for example, U.S. Pat. No. 5,396,247 (JP 3,064,644). In this A/D converter circuit, a plurality of inverting circuits is connected like a ring to constitute a pulse circulating circuit. An input voltage which is an analog input is applied as a power supply voltage to each of the inverting circuits to convert the input voltage into a digital data by utilizing the fact that an inverting operation time of the inverting circuits varies depending upon the power supply voltage.
However, according to the A/D converter circuit of this type, even if the input voltage is linearly varying, the A/D-converted output (digital data) varies non-linearly since the delay time of the inverting circuits constituting the pulse circulating circuit does not linearly vary relative to a change in the power supply voltage.
According to a method of correcting non-linearity and an apparatus for correcting non-linearity of the A/D-converted output data, as disclosed in U.S. Pat. No. 6,891,491 (JP 2004-274157A), an approximated straight line or curve is calculated or corrected based on a digital operation to suppress variation in the digital data caused by non-linear delay characteristics for a change in the power supply voltage.
According to the latter prior art, however, it is necessary to repeat a predetermined arithmetic operation for every A/D conversion to digitally operate the approximated straight line or curve. Therefore, even if a variation in the digital data caused by non-linear delay characteristics of the inverting circuit could be suppressed, the scale of the arithmetic processing circuit increases and, besides, an extended period of time is required for the operation that is complex and that occurs repetitively. Thus, this technology is not suited for high-speed A/D conversion processing.
The above problem may be technically solved by utilizing a GaAs device capable of executing the arithmetic operation at high speeds for the arithmetic processing circuit. However, the cost of the GaAs device itself and the presence of the pulse circulating circuit that does not require high-speed processing necessitate a new step in the process for producing semiconductors. This will increase the cost of production.